May 19, 2011

The importance of room damping

Room damping or more specifically bass damping, is an essential part of accurate bass reproduction. All charts shown here are based on a REW simulation of a single subwoofer in the left corner in a small room 4.65m wide x 3.9m deep x 2.85m high. You quickly and easily generate similar results. They will differ based on the chosen listening position. Here we are focusing on the room itself, without any acoustic treatment.

What is bass damping?

Bass damping is simply absorption in the bass range. I refer to it as bass damping as we are dealing with room resonances that need to be damped. You can achieve it with room construction or added treatment. Here we will focus on the former. Plasterboard/drywall light framed walls work well as bass traps. Sound waves cause the boards to flex and in the process, energy is absorbed. The effectiveness is based on the depth of the air gap behind, the presence of insulation in the cavity and the mass and stiffness of the membrane.

Here you can see the impact of damping on the frequency response.

The top navy blue line is a room with minimal damping. That means a very low absorption coefficient of 0.1. You can see this is a poorly performing room with the worst peaks and dips. The tell tale sign is that they are all narrow Q (sharp and narrow). The best room has an enclosure with very high absorption coefficients (0.9). Each chart increases the absorption coefficient in increments of 0.1. A coefficient of 0.1 will absorb 10% of a sound wave. The black line represents the kind of damping you might see in an anechoic chamber. Obviously it isn't practical as a listening room due to likely cost and pragmatic issues. This level of damping would only be achieved with a great deal of added acoustic treatment.

Here are three selected rooms.

The navy blue line again shows the worst case scenario. The magenta line shows a room that is quite good and may represent many lightly constructed rooms with plasterboard/drywall and a timber floor. The green line shows a room that is very highly damped. It would probably require added bass traps, or special construction. 

In this waterfall, you can see just how bad this room is. Not only are the peaks very sharp, but they also decay at a very slow rate. After 300 ms they have only decayed by 10 dB. While this is a problem in the bass range, it is even worse above 100 Hz where the ear is more sensitive to time domain effects. The result would be a cavernous sound. Fortunately few rooms would be this bad.

In this waterfall, you can see much better performance. Here one of the limits of the simulation becomes clear. In a real room, the bass region below 80 Hz would be worse, but above it would probably be better. Still, it isn't too difficult to see how this is a big improvement.
With very high damping in this room you can see that the response is much flatter and any peaks and dips are much broader. The decay is very even and rapid. If a room were built to perform this well (not likely), very little extra effort would be required for exceptional performance. Unfortunately this level of damping requires a very well designed room and extensive added bass traps.

Bass traps vs room modification

It is important to combine acoustic treatment with a room that also provides damping within its envelope. The room damping tends to be restricted in bandwidth and is not likely to do all that is required. If it is left out, the additional space required for larger bass traps may not be practical. The best and most sensible approach is to combine the two different types of damping. With this approach in mind, the bass traps used should be of the porous broadband type. Membrane-based traps are not likely to have the required bandwidth.


  1. What program did you use to make the waterfall plot and what microphone?

  2. The answer is here:


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